The long range goals of this project are to determine the role of I-region associated (Ia) antigens in the regulation of immunocompetent cell interactions and to determine the mechanism of action and site of expression of the histocompatibility linked immune response (Ir) genes. During the past year studies have been performed in three major areas: 1) we have demonstrated that the binding of large amounts of anti-antigen antibody to the macrophage surface results in marked inhibition of T cell proliferation and that this result is consistent with the view that "processed" antigen is displayed on the cell surface perhaps in association with Ia antigens; 2) using a model system in which T cell stimulation is induced by active aldehydes (CHO) on the macrophage surface, we have shown that Ia antigens play a critical role in CHO induced T cell proliferation and may function as triggers or second signals in this process; 3) we have shown that monoclonal antibodies to guinea pig Ia antigens react with different functional parts of Ia molecules and that certain parts of an Ia molecule participate in the presentation of certain antigens while other regions of the same molecule present different antigens.